This disclosure relates to optical film coatings. Specifically, this disclosure relates to optical film coatings that reduce interference colors known as Newton rings.
Optical displays typically have an exposed viewing surface made from a thermoplastic film or substrate. Commonly employed thermoplastic polymers have very good optical transparency, dimensional stability and impact resistance, but unfortunately have poor abrasion resistance. The optical displays of devices such as personal digital assistants (PDAs), cell phones, liquid crystal display (LCD) panels, touch-sensitive screens and removable computer filters are subjected to frequent handling and contact with the user's face or fingers, styli, jewelry and other objects. For example, facial oils can adversely affect contrast, color saturation or brightness of a cell phone display. The screens of projection televisions and laptop computers are handled less frequently but nonetheless are susceptible to being touched, scratched or smudged. Consequently, the viewing face of the display is susceptible to scratches, abrasion and smudges arising during routine use. This can cause the display to lose resolution and clarity, and sometimes to become unreadable or inoperative. To protect such displays, protective films or coatings can be employed.
Hard coats have been used to protect the face of optical displays. These hard coats typically contain inorganic oxide particles, e.g., silica, of nanometer dimensions dispersed in a binder precursor resin matrix, and sometimes are referred to as “ceramers”. These hard coats can have a thickness in a range of 1 to 15 μm. Current hard coat films produce optical interference fringes attributable at least in part to non-uniform thickness of the hard coat layer.
When light enters a hard coated display, the reflectivity of the light undergoes a period change according to the wavelength of the incident light. The reflection spectrum with the aforementioned specific repeating period is referred to as an interference fringe. Attempts have been made to reduce or eliminate these interference fringes. The amplitude of the fringe may depend on the difference in the index of refraction between the substrate and the hard coat layer. The greater the difference in the index of refraction between the substrate and the hard coat layer, the greater the amplitude and contrast of the interference pattern.
In the past, in order to eliminate the aforementioned interference pattern, several methods described below have been used. The first method is to increase the hard coat film thickness significantly. When a thick hard coat layer is used, the frequency of the fringes becomes smaller and the color change becomes insignificant even when the thickness of the hardcoat layer varies from area to area. However, a thick hard coat presents new problems. First, in order to cover the interference pattern, it is necessary to coat a hard coat layer a thickness of at least 20 μm to 30 μm. As a consequence, shrinkage of the hard coat layer at the time of curing is increased significantly and coating is difficult and the cost is increased significantly. Furthermore, cracks are likely to form in the hard coat layer as well.
A second method is to match the index of refraction of the substrate with the index of refraction of the hard coat layer. For example, in general, the index of refraction of the hard coat layer is in the range of approximately 1.49 to 1.55; thus, when a TAC film with an index of refraction of 1.49 is used for the substrate, the amplitude of the fringes becomes very low since the index of refraction of the substrate and that of the hard coat layer are essentially the same and contrast of the interference pattern is reduced. However, in comparison to polyethylene terephthalate (PET) film, the cost of TAC film is considerably higher, and furthermore the film itself is soft and rupturing occurs easily. Because of the soft material used for the substrate itself, craters are likely to form at the time of coating and surface flaws are likely to occur. In addition, the TAC film underneath the hard coat layer is soft, thus, the pencil hardness is reduced even with the hard coat layer.
A third method is to match the index of refraction of the hard coat layer with the index of refraction of the substrate, since the substrate may be limited by the particular application. For example, an appropriate amount of one or more metal oxide super-fine particles with high index of refraction selected from the group consisting of ZnO, TiO2, CeO2, Sb2O5, ITO, In2O3, Y2O3, La2O3, Al2O3, HfO2 and ZrO2 are mixed with a binder made of a thermosetting resin and/or an ionizing radiation curable type resin to form a hard coat layer, and the index of refraction of the aforementioned hard coat layer is brought closer to the index of refraction of the PET film (in general, approximately 1.65). However, this technique may cause many problems. First, in general, the metal oxide super-fine particles are materials with color and when the hard coat layer is coated with a film thickness of at least 3 μm to satisfy the performance of the hard coat, in many cases, the coating is colored. When toning is done to eliminate the color (to achieve specific transmittance at each wavelength) overall transmittance is reduced. Furthermore, when super-fine particles other than silica are included, hardness is reduced in comparison to the case where a hard coat layer alone is used. When the index of refraction of the hard coat layer of the outermost layer is increased, reflectivity is increased. As a result, the transmittance required for transparent optical material cannot be achieved.
As described above, the greater the difference in the index of refraction between the substrate and hard coat layer, the greater the amplitude and contrast of the interference pattern. When the contrast of the aforementioned interference pattern is high, the viewer can experience significant discomfort.